1. Field of the Invention
The present invention relates to a single-end transmission line for transmitting analog radio-frequency signals of microwave band, millimeter-wave band or the like or digital signals, and further relates to a radio-frequency circuit which contains such a transmission line.
2. Description of the Related Art
FIG. 18A shows a schematic cross-sectional structure of a microstrip line which has been used as a transmission line in such a conventional radio-frequency circuit as shown above. As shown in FIG. 18A, a signal conductor 103 is formed on a top face of a board 101 made of a dielectric or semiconductor, and a grounding conductor layer 105 is formed on a rear face of the board 101. Upon input of radio-frequency power to this microstrip line, an electric field arises along a direction from the signal conductor 103 to the grounding conductor layer 105, and a magnetic field arises along such a direction as to surround the signal conductor 103 perpendicular to lines of electric force. As a result, the electromagnetic field propagates the radio-frequency power in a lengthwise direction perpendicular to the widthwise direction of the signal conductor 103. In addition, in the microstrip line, the signal conductor 103 or the grounding conductor layer 105 does not necessarily need to be formed on the top face or the rear face of the board 101, but the signal conductor 103 or the grounding conductor layer 105 may be formed within the inner-layer conductor surface of the circuit board on condition that the board 101 is provided as a multilayer circuit board.
Since transmission of a radio-frequency signal along the microstrip line involves a distribution of radio-frequency magnetic fields around the transmission line, there arises unwanted radiation of electromagnetic waves. Whereas a structure in which grounding conductors are placed on both sides of a signal conductor to make an electromagnetic shielding from the external field as in strip lines makes it possible to suppress the unwanted radiation to some extent, it is impossible, in principle, for microstrip lines to suppress the unwanted radiation because the microstrip line has the grounding conductor only on one side of the board.
The above description has been made on a transmission line for use of transmission of single-end signals. However, as shown in a sectional view of a line structure in FIG. 18B, it becomes possible to reduce unwanted radiation when two microstrip line structures 103a, 103b are placed in parallel on a top face of a board 101 made of a dielectric or semiconductor, and a grounding conductor layer 105 is formed on a rear face of the board 101 so that 103a and 103b are used as differential signal transmission lines by with signals of opposite phases transmitted through the lines, respectively. However, in this case, there arises a problem that the circuit occupation area increases because of the need for paired signal conductors. Also, whereas radio-frequency signals are not superimposed in principle in a bias line for supplying a bias to active elements within the circuit, insufficient processing in the circuit may cause leakage of the radio-frequency signals, which may cause unwanted radiation. The bias line, which is a line for DC current supply, would not be suitable with a differential structure. That is, since it is inevitably necessary for the bias line to be a microstrip line structure, there arises a need for a structure for reducing unwanted radiation.
Now, the principle of occurrence of unwanted radiation is explained by using a schematic perspective view of a typical transmission line shown in FIG. 19. A linear transmission line 291 is so constructed that a grounding conductor 105 formed on a rear face of a dielectric substrate 101 serves as its grounding conductor part and one signal conductor placed linearly on a top face 281 of the dielectric substrate 101 serves as its signal conductor part. Assuming that both ends of the transmission line 291 are terminated by unshown resistors, respectively, radio-frequency circuit characteristics of the one transmission line 291, i.e. the origin of unwanted radiation in this case, can be understood by substituting a current-flowing closed current loop 293a for the transmission line 291. As shown in FIG. 19, due to a radio-frequency current 853 that has flowed through the current loop 293a, a radio-frequency magnetic field 855 is induced so as to extend through the current loop 293a, causing radiation due to the radio-frequency magnetic field 855 to be generated. The closed loop has an area indicated by the label A. In this case, since the intensity of the radio-frequency magnetic field 855 is proportional to a loop area A of the current loop 293a, there holds a proportional relationship between the loop area A of the current loop 293a and a radiation electric field strength E. Moreover, a proportional relationship holds also between the square of the frequency f of the radio-frequency current and the radiation electric field strength E, and moreover a proportional relationship also holds between the current amount l of the flowing radio-frequency current and the radiation electric field strength E. That is, in a radio-frequency circuit, there is a tendency that increasing transmission line length causes the loop area A to increase more and more so that the unwanted radiation also increases, and further that higher-speed signals transmitted as well as increased current amounts cause unwanted radiation to increase.